Currently, three-way or four-way spool valves are used to control the pressurization and venting of pneumatic cylinders. Spool valves require extremely close tolerances and are therefore difficult to manufacture and assemble and are susceptible to becoming jammed within their associated valve bore during use. When jammed, the valve bore in the body and/or the spool valve itself may become damaged thereby necessitating repairing or replacing one or both of them.
Because of the close tolerances required for the operation of spool valves, they are difficult to assemble into their associated valve bores and are extremely intolerant of particulate contaminants and wear. When contaminants enter the associated valve bore of a spool valve, the spool valve or valve bore can become damaged which requires repairing or replacing the spool valve and/or the entire spool valve assembly. The spool valve can also become jammed within the valve bore due to contaminants which requires repairing or replacing the spool valve assembly which can be labor intensive and costly. Further, when used in an assembly line environment, failure of the spool valve assembly requires temporarily shutting down the assembly line to repair or replace the spool valve assembly which is time consuming and very costly.
One current use of a spool valve is to control the welding cylinders, in an automated system such as used in the automotive industry to drive a welding gun of an electrical resistance welder. The spool valve controls the pressurization of the drive cylinder which in turn controls the weld clamp jaws of the welding gun which, when clamped provides the weld to the workpiece in the clamp. Typically, this operation is automated and within an assembly line and thus, if the spool valve becomes jammed the jaws of the weld clamp can remain engaged with the workpiece and can destroy expensive parts, such as automotive bodies as well as other components of the assembly line. This also requires shutting down the assembly line to repair or replace the spool valve assembly and disengage the weld clamp jaws from the workpiece. Shutting down the assembly line is extremely costly and time consuming and repairing or replacing the spool valve assembly is labor intensive and therefore also very costly.
Further, the spool valve is only capable of responding to an on-off signal and does not provide a force which is proportional to an input signal and therefore does not provide a force which can be varied to vary the clamping force and subsequent welding force of the weld clamp. To perform this function with a spool valve, a pressure regulator must be added to the spool valve assembly which increases the complexity of the assembly and also its cost.